STABILITY RELATIONS OF JAROSITE AND NATROJAROSITE AT 150-250-DEGREES-C

The jarosite decomposition reaction KFe3(SO4)2(OH)6 = 1.5Fe2O3 + K+ 2SO42- + 3H+ + 1.5H2O and the analogous natrojarosite reaction have be en studied in hydrothermal experiments of 1 to 11 weeks duration at 15 0-250-degrees-C. For a log mK2SO4 in the coexisting aqueous phase of b etween -0.4 and -1.2, the log mH2SO4 required to stabilize jarosite de creases from -0.35 +/- 0.05 at 250-degrees to -0.58 +/- 0.12 at 200-de grees-C. At higher K2SO4 concentrations the H2SO4 concentration requir ed to stabilize jarosite increases at both temperatures, consistent wi th the position of the jarosite-hematite boundary predicted with PHRQP ITZ. Natrojarosite could not be produced from hematite at 250-degrees- C. At 200-degrees-C, the natrojarosite-hematite boundary occurs at log mH2SO4 of -0.17 +/- 0.08 at log mNa2SO4 of -0.2 to -1.0. At log mNa2S O4 below about -1.0, natrojarosite is unstable with respect to the ass emblage hematite + Fe(SO4)(OH). Apparent standard molal Gibbs free ene rgies computed from the experimental data are DELTA(jarosite, 200-degr ees, 100 bars)0 = -3416.3 +/- 1.7 kJ/mol and DELTAG(natrojarosite, 200 -degrees, 100 bars)0 = -3371.9 +/- 2.0 kJ/mol. The estimated errors re flect a lack of complete reversibility in the experiments, but do not consider other, potentially greater, sources of uncertainty such as th ose associated with calculation of aqueous activity coefficients. Addi tional experiments were used to study the exchange reaction KFe3(SO4)2 (OH)6 + Na+ = NaFe3(SO4)2(OH)6 + K+. Ln K(D) for this reaction is 3.1 +/- 0.5 at 250-degrees-C, -3.7 +/- 0.4 at 200-degrees-C, and -4.9 +/- 0.2 at 150-degrees-C. Ln K(D) does not show a systematic variation wit h X(Na) at either 250 or 200-degrees-C, which suggests that jarosite a nd natrojarosite may be modeled as an ideal solid solution at these te mperatures. At 150-degrees-C In K(D) does not vary from X(Na) = 0.1 to 0.6, but good constraints on its value could not be obtained outside this compositional range. The experimental data therefore do not rule out a significant departure from ideality at higher X(Na) at 150-degre es-C.